Vehicles with hybrid drive systems have become one important way of reducing vehicular emissions and improving fuel economy. Various specific hybrid drive systems have been proposed and/or built and many of these hybrid drive systems employ a start-stop strategy wherein the internal combustion engine in the vehicle is stopped when not needed and is re-started as needed.
While such start-stop strategies make a significant contribution to achieving the reduced emissions and improved fuel economy goals of hybrid vehicles, the implementation of start-stop strategies suffers from several problems.
One of these problems is the fact that the conventional starter motor and ring gear starting system which is commonly employed on most internal combustion engines is not designed to survive the much higher number of starter cycles expected over the lifetime of a hybrid drive system. Specifically, a typical starter motor and ring gear starting system is designed to reliably perform as many as fifty thousand starting cycles over the lifetime of a vehicle. In contrast, it is estimated that the internal combustion engine in a hybrid vehicle employing a start-stop strategy must be started as many as eight hundred thousand times over the lifetime of the vehicle and thus conventional starter and ring gear systems can be woefully inadequate.
In an attempt to avoid failures of the conventional starter and ring gear system, some hybrid drives employing start-stop strategies have been equipped with an alternator/motor on their accessory drive (either a Front Engine Accessory Drive—FEAD, or a Rear Engine Accessory Drive—READ). With such systems, the conventional starter motor and ring gear is used to start the internal combustion engine in conditions requiring high starting torques (such as starting of the vehicle before normal operating temperatures have been achieved) but is re-started after a short term shutdown (or when the vehicle is otherwise at normal operating temperatures) with the alternator/motor which generates the necessary torque to turn the crankshaft of the engine and that torque is transferred from the alternator/motor to the crankshaft by the accessory drive belt.
While such alternator/motor systems do work, they also suffer from disadvantages. In particular, as the torque necessary to rotate the crankshaft and start the internal combustion engine can easily exceed one hundred and forty Newton meters, the accessory belt must be much stronger than would otherwise be required and the tension of that belt also must be much higher than would other wise be required. Further, to achieve the necessary belt strength, the belt must typically be thicker and/or wider, thus occupying more space which may not be readily available.
Due to the required higher tension in the accessory drive belt, each of the accessories on the accessory drive must have more robust bearings and mounting hardware, increasing the manufacturing cost of the vehicle, and parasitic losses in the belt, which result in fuel economy decreases, are increased with the tension and with the associated increased stiffness of the belt.
Further, as the accessory drive is tensioned in one direction when the internal combustion engine is running and is tensioned in the other direction when the internal combustion engine is being started by the alternator/motor, the accessory drive design will require special purpose idlers and/or tensioners and may require multiple instances of such tensioners and/or idlers, all of which further increases the manufacturing expense of the vehicle.
Another problem which exists with hybrid drives that employ start-stop strategies is that, when the internal combustion engine is stopped, the accessory drive system is not operating. Thus, accessories powered by the accessory drive, such as the power steering pump, air conditioning compressor, alternator, etc. are not operating. The non-operation of these accessories can reduce the comfort level of the occupants of the vehicle and, in the case of the power steering pump, can lead to unsafe operating conditions.
It is desired to have a method and system for re-starting and/or operating vehicles with hybrid drives that employ start-stop strategies.